THE OrERCULUM 



267 



Fig. 181. — Ehvrna spirata 

 Lam., E. Indie.s. F, 

 foot ; OP, oiiereuliim ; 

 P, penis ; S, siphon ; 

 T, tentacles, with eyes 

 at their base. (After 

 Sonleyet. ) 



The Operculum. — The operculum is a cuticular development 

 of a group of cells situated on the dorsal 

 side of the foot, exactly over the teriuinal 

 point of the filH'es of the columellar muscle. 

 It is so situated that in crawling it is 

 generally carried free of the shell, some- 

 times at the extreme upper end of the foot, 

 more usually somewhat nearer to the shell 

 (Fig. 181). In Ptcrocydas it is pushed Ijack 

 into the umbilicus when the aiiimal is iu 

 motion. 



The opercidum is present in nearly all 

 land, fresh- water, and marine Prosohran- 

 chiata, absent in all Opisthobranchiata in 

 the adult state, except Actacon, and in all 

 I'Tilmonata, except Aniphihola. It has been 

 lost in the following marine Prosobran- 

 chiata : many Cancellariidae and Conidae, 

 Olica (though present in Olvvella and 

 AncUJa), Harpidae, Marginellidae, Voluta proper (though pre- 

 sent in V. musica), nearly all Mitridae, Cypraeidae, Doliidae, 

 lanthinidae ; and, of land genera, in Proserpinidae. It is evident, 

 therefore, that its presence or absence is of limited value in 

 classification. In some species of Ampullaria and Katica it 

 is horny, in others shelly. Dall fouiul that in a nund^er of 

 specimens of Vulvtharpa ampullacea, 15 p.c. had opercula, 10 

 p.c. traces of the operculigenous area, Ijut no operculum, the rest 

 no trace of either. Monstrosities of Buccinum undatiivi some- 

 times occur, wdiich have two, or in rare cases three opercula. 



As a rule, the operculum exactly fits the mouth of the shell. 

 P)Ut in cases where the mouth is very large (e.g. Conns, Stromhns, 

 ConcJwlepas, some Bnllia), it only covers a very small portion 

 and is (juite inadequate as a protection (Fig. 62, p. 155). Again, 

 when the shell has assumed a more or less limpet-shaped form, 

 and habitually adheres to fiat surfaces without much occasion for 

 locomotion, the operculum becomes degraded and is pro1)ably on 

 the way to being lost altogether. This is the case with Navi- 

 celld (a modified JYerita, see Fig. 13, p. 17), Concholepas (a modi- 

 fied Fnrpnra), Sigaretus (a modified Natica). Probably the nu)re 

 completely patelliform shells of Crepidnla, Haliotis, Fissitrclla, 



